Holographic recording system and optical chopper

ABSTRACT

A holographic recording system with a simple structure is provided that can make a holographic recording beam follow a recording medium traveling even at high speed. The holographic recording system  10  includes a chopper lens  30  that travels in synchronization with and in a same direction as the holographic recording medium  14.  The chopper lens  30  is formed so as to move the axes of incident information and reference beams relative to the recording medium and the chopper lens, and to irradiate them to an identical information recording region  15  in the holographic recording medium  14.

TECHNICAL FIELD

The present invention relates to a holographic recording system and anoptical chopper that utilize holography to record information into aholographic recording medium.

BACKGROUND ART

In holographic recording that utilizes holography to record informationinto a recording medium, an information beam and a reference beam aresuperposed each other in a holographic recording medium and an opticalinterference pattern formed by these beams is written as a diffractiongrating.

As one of holographic recording systems, there is a system thatmodulates a recording beam comprising the information and referencebeams in time and space and irradiates it to a recording medium whilecontinuously moving the recording medium with respect to the opticalaxis of the recording beam. This includes, for example, a holographicrecording system that irradiates the recording beam from an optical headwhile rotating a disc-shaped recording medium.

In this case, to prevent an aberration of the diffraction grating to berecorded in the recording medium moving relative to the recording beam,a light pulse as short as several tens nanoseconds is necessary toperform recording, causing the sensitivities in both recording andreproducing to decrease.

Alternatively, as described in, for example, Japanese Patent Laid-OpenPublication No. 2002-183975, there is another holographic recordingsystem that drives an optical head along tracks formed in thecircumference direction of a disc-shaped recording medium by using anelastic arm, an electromagnetic coil, and a magnet to change theposition of the recording beam.

This is the so-called stop-and-go recording method, in which recordingis performed so that the variation of the relative position of therecording beam with respect to an information recording region in therecording medium will not occur when the recording medium rotates, andthen the optical head is moved back to the original position while therecording beam (laser beam) turns off.

In the holographic recording system described in the above JapanesePatent Laid-Open Publication No. 2002-183975, irradiation positionmoving means comprising an elastic arm, an electromagnetic coil, and amagnet is provided as described above and is controlled by a follow-upcontrol circuit so as to make the irradiation position of the recordingbeam follow an information recording region in the recording medium fora predetermined time and move it back to the original position.Therefore, the structure is complicated, the fabrication cost is high,and there is a risk that the recording beam cannot reliably follow therecording medium when the recording medium is rotated at high speed.

DISCLOSURE OF THE INVENTION

The present invention addresses the above-described problems in theconventional technology, with an object of providing a holographicrecording system and an optical chopper that have a simple structure andcan make the record beam reliably follow a recording medium even if therecording medium is moved at high speed.

As a result of diligent research, the present inventor has found thatholographic recording is possible so that the recording beam can beirradiated to an identical position in the holographic recording medium,by using a chopper lens traveling in synchronization with theholographic recording medium and deflecting a recording beam accordingto its incident positions on the chopper lens.

In summary, the above-described objectives are achieved by the followingaspects of the present invention.

(1) A holographic recording system, comprising:

a light beam irradiation unit for irradiating an information beam and areference beam so that an interference pattern formed by the informationbeam and the reference beam can be recorded as a diffraction gratinginto an information recording region in a holographic recording medium;a recording medium moving unit for moving the holographic recordingmedium with respect to the optical axes of the information beam and thereference beam; and an optical chopper provided on the optical axes ofthe information beam and the reference beam between the light beamirradiation unit and the holographic recording medium, the opticalchopper moving the optical axes in a same traveling direction as, bysubstantially a same traveling distance as, and in synchronization withthe information recording region, when performing holographic recordinginto the information recording region by using the information beam andthe reference beam.

(2) The holographic recording system according to (1), wherein: theoptical chopper comprises a chopper lens that travels in synchronizationwith and in a same direction as the information recording region; andthe chopper lens is formed so as to irradiate the information beam andthe reference beam from the light beam irradiation unit to be incidenton the information recording region, and to refract the information beamand the reference beam according to variations of incident positions onthe chopper lens to make exit optical axes pass through an identicalpoint, when performing the holographic recording.

(3) The holographic recording system according to (2), wherein: theholographic recording medium has a disc shape; the optical choppercomprises a disc-shaped chopper disc having substantially a same radiusas the holographic recording medium and capable of rotating insynchronization with the holographic recording medium; the chopper dischas a plurality of ring-shaped regions disposed at different positionsin a radius direction; and the chopper lenses are discretely disposed ineach of the ring-shaped regions in the chopper disc, with identicallength and pitch in a circumference direction, and are formed so as tomake the reference beam and the information beam pass through thechopper disc in a thickness direction.

(4) The holographic recording system according to (2) or (3), whereinthe chopper lens is a cylindrical lens elongated in a directionorthogonal to a traveling direction of the holographic recording medium.

(5) The holographic recording system of either one of (3) or (4),wherein the holographic recording medium and the chopper disc arecoaxially and integrally provided so as to be freely rotated.

(6) The holographic recording system of either one of (1) to (5),further comprising: a CCD camera for forming a reconstructed holographicimage by a diffraction beam from the information recording region in theholographic recording medium, when a reconstructing beam is irradiatedto the information recording region through the chopper lens; and acompensating unit for compensating position offset of the reconstructedholographic image obtained by the CCD camera, caused by movement of thechopper lens.

(7) An optical chopper comprising: a chopper disc having a rotatabledisc shape and comprising a plurality of ring-shaped regions disposed atdifferent positions in a radius direction; and a plurality of chopperlenses that are discretely disposed in the ring-shaped regions in thechopper disc, with an identical length and pitch in a circumferencedirection, and refracts and transmits a beam incident from one surfaceof the chopper disc to the other surface thereof, wherein the chopperlens is formed such that, when the chopper lens is rotated in a fixeddirection at a constant speed together with the chopper disc, an exitoptical axis of a laser beam incident from the one surface is refractedso as to take a fixed position in a rotational direction in a specificplane disposed on the other surface.

(8) The optical chopper of (7), wherein the chopper lenses are each acylindrical lens elongated in the radius direction of the chopper disc.

(9) The optical chopper of (7) or (8), wherein the chopper lenses andthe chopper disc are integrally formed of a transparent resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view including part of an optical blockdiagram and illustrating the holographic recording system according toan exemplary embodiment of the invention.

FIG. 2 is a plane view illustrating the optical chopper used in theexemplary embodiment.

FIG. 3 is an enlarged perspective view of portion III in FIG. 2.

FIG. 4 illustrates optical path diagrams describing the operation of achopper lens in the optical chopper.

FIG. 5 is a schematic perspective view including part of a block diagramand illustrating the process of reproducing information in theholographic recording system.

BEST MODE FOR CARRYING OUT THE INVENTION

An exemplary embodiment of the invention will now be described in detailwith reference to the attached drawings.

As shown in FIG. 1, a holographic recording system 10 according to theexemplary embodiment of the invention comprises: an optical head 12 thatis a light irradiation unit; a holographic recording medium 14; and anoptical chopper 16 that is an optical axis moving unit. An interferencepattern formed by information and reference beams from the optical head12 is recorded in the holographic recording medium 14 as a diffractiongrating. The optical chopper 16 is provided on the optical axes of theinformation and reference beams between the optical head 12 and theholographic recording medium 14, and functions as an optical axisshifting unit which moves the optical axes while making theseinformation and reference beams be in synchronization with the movementof the holographic recording medium 14.

The optical head 12 comprises: a laser light source 18; a beam expander20 for expanding the diameter of the laser beam from the laser lightsource 18; a beam splitter 22 for splitting the laser beam, the diameterof which has been expanded by the beam expander 20, into a referencebeam and an information beam; an information optical system 24 for theinformation beam split by the beam splitter 22; and a reference opticalsystem 26 for the reference beam.

The information optical system 24 comprises: a mirror 24A that reflectsthe information beam having passed through the beam splitter 22; aFourier lens 24B that irradiates the information beam reflected by themirror 24A to an information recording region 15 (see FIG. 4) in theholographic recording medium 14; and a spatial light modulator (SLM) 24Cthat is provided on the optical path of the information beam between themirror 24A and the Fourier lens 24B and adds two-dimensional informationto the information beam.

The reference optical system 26 comprises a mirror 26A for reflectingthe reference beam reflected by the beam splitter 22 and a Fourier lens26B for irradiating the reference beam reflected by the mirror 26A tothe information recording region 15.

Referring to FIG. 2, the optical chopper 16 comprises a rotatabledisc-shaped chopper disc 28 and a plurality of chopper lenses 30 thatare integrally provided in the chopper disc 28 and refracts andtransmits the beams incident from the upper surface side of the chopperdisc 28 to the lower side in FIG. 1.

The chopper lenses 30 are discretely disposed in a plurality of ringregions 32A, 32B, 32C, and so on, which are disposed at differentpositions in the radius direction of the chopper disc 28, with identicallength and pitch (p) in the circumference direction.

As shown in an enlarged view in FIG. 3, the chopper lens 30 is obtainedby cutting out part of a cylindrical lens to a width w.

In the exemplary embodiment, the optical chopper 16 is formed of thesame optical resin as used in precision lenses such as aspherical lensesto integrate the chopper disc 28 and the chopper lenses 30. As anoptical resin, for example, polycarbonate resin or acrylic resin isused.

As described above, the chopper lenses 30 are discretely formed in thecircumference direction of the ring regions 32A, 32B, 32C, and so on inthe chopper disc 28. In addition, masks 33 shown by hatched lines inFIG. 2 are also formed between each of the chopper lenses 30 by, forexample, printing to block the beams.

The optical chopper 16 is supported by a spindle controller 34 so as torotate integrally with the holographic recording medium 14. In FIG. 2,the chopper lenses 30 disposed in the ring region 32A are only shown,and those in other ring regions are omitted.

The chopper lens 30 is formed so that when it is rotated in a certaindirection together with the chopper disc 28, the optical axes of thelaser beams (information and reference beams) incident from the upperside in FIG. 1 can take a fixed position in the rotational direction inthe holographic recording medium 14.

That is, as shown in FIGS. 4(A) to 4(C), when the chopper lens 30rotates and thereby moves together with and in synchronization with theholographic recording medium 14 in the right direction in the drawing,the information and reference beams having passed through the respectiveFourier lenses 24B and 26B are incident from the upper side onto thechopper lens 30, from which they exit downward. In this case, the lensshape is formed so that their optical axes can take a fixed position inthe rotational direction in a specific plane of the holographicrecording medium 14 and thereby the beams illuminate an identicalinformation recording region 15 in the holographic recording medium 14.

More in detail, as shown in FIG. 4(A), the optical axis of the beamincident from the right side of the chopper lens 30 in the drawing isrefracted toward the left by the chopper lens 30, and is irradiated tothe information recording region 15.

Next, when the holographic recording medium 14 and the chopper lens 30move as shown in FIG. 4(B), the optical axes of the information andreference beams on the exit side match the center axis of the chopperlens 30, and are irradiated to the information recording region 15moving in synchronization with the chopper lens 30.

When the chopper lens 30 further moves in the right direction as shownin FIG. 4(C), the information and reference beams are incident on theleft side of the chopper lens 30 rather than the center thereof, wherethey are refracted toward the right and irradiated to the informationrecording region 15 in the synchronously moving holographic recordingmedium 14.

Accordingly, while the holographic recording medium 14 and chopper lens30 are moving in synchronization with each other, the information andreference beams from the Fourier lenses 24B and 26B are irradiated tothe same information recording region 15.

Incidentally, the optical head 12 is typically formed so that it canintegrally move in the radius direction of the holographic recordingmedium 14.

In the optical head 12, a CCD camera 40 is provided on the opposite sideof the optical chopper 16 with the holographic recording medium 14 inbetween so that a reconstructed holographic image can be formed when thediffraction beam is incident from the information recording region 15via a Fourier lens 41.

When forming the reconstructed holographic image, the beam from theinformation optical system 24 is blocked by the spatial light modulator24C to irradiate the reference beam from the reference optical system 26to the information recording region 15, the diffraction beam from whichis then made incident on the CCD camera 40 through the Fourier lens 41(see FIG. 1).

As shown in FIG. 5, a compensating unit 42 is connected to the CCDcamera 40. When reproducing information, the compensating unit 42compensates the position offset of a reconstructed holographic image,which is caused by movement of the reconstructed holographic image onthe CCD camera 40, the movement occurring due to the rotation of theholographic recording medium 14 and chopper lens 30. The compensatedreconstructed holographic image is then output to a memory unit 44and/or a display unit 46.

The necessity and operation of the compensating unit 42 will bedescribed in detail below.

When an information beam is irradiated to the center of the chopper lens30 (solid line), it exits toward a position A on the holographicrecording medium 14 without changing its optical axis. When the opticalchopper 16 and the holographic recording medium 14 rotate in thedirection indicated by an arrow and the information beam is irradiatedto the left side of the chopper lens 30 as indicated by a broken line,the information beam is diffracted by the chopper lens 30 toward theright in FIG. 5 and exits toward a position B on the holographicrecording medium 14. In this case, since the traveling distances of theoptical chopper 16 (chopper lens 30) and the holographic recordingmedium 14 by rotation and the distance between the positions A and B areall substantially equal to one another, the information beam passingthrough an identical chopper lens 30 always illuminates an identicalpoint on the holographic recording medium 14.

In contrast, as appreciated from FIG. 5, a position on the CCD camera 40toward which an image is focused changes as the optical chopper 16 andthe holographic recording medium 14 rotate. In this case, however, itschanging rate (traveling speed of the reconstructed image on the CCDcamera) is predictable if the arrangements of the optical systems andthe holographic recording medium 14, the rotational speed of theholographic recording medium 14 (the optical chopper 16), and theposition (radius) of the chopper lens 30 in the optical chopper 16 areknown. Therefore, the same reproduced image as the one obtained when theholographic recording medium 14 is stationary can be easily obtained byperforming image processing (translational movement) by using thecompensating unit 42 on the reconstructed image detected by the CCDcamera 40.

In the above-described exemplary embodiment, the chopper disc 28 and thechopper lenses 30 are integrally formed of a transparent resin, but thepresent invention is not limited to this formation. Chopper lenses mayalso be formed so as to be mounted on a chopper disc. When the chopperdisc 28 and the chopper lenses 30 are integrally formed as in theexemplary embodiment, however, the fabrication cost and the accuracy ofassembling the chopper lenses and chopper disc can be improved.

According to the above exemplary embodiment, the chopper lens 30comprises a cylindrical lens or a part thereof elongated in the radiusdirection of the chopper disc 28, so that, as shown in FIG. 1, it fitsto the information and reference optical systems 24 and 26 disposedabove an identical radius of the holographic recording medium 14 and inthe plane orthogonal thereto, more specifically to the arrangement ofthe optical axes of the information and reference beams passing throughthe Fourier lenses 24B and 26B.

In the above exemplary embodiment, the holographic recording medium 14is constructed so as to rotate, but the invention is not limited to thisconstruction. Provided that it moves in the orthogonal direction withrespect to the plane defined by the optical axes of the information andreference beams, the movement is not restricted to rotation. In thiscase, however, the chopper lenses 30 are also moved in synchronizationwith and in the same direction as the holographic recording medium 14.

INDUSTRIAL APPLICABILITY

Since the invention has the above-described structure, it has anexcellent advantage in that the axes of the information and referencebeams can be moved following the holographic recording medium, withoutemploying any irradiation position moving means having a complicatedstructure for the optical head.

1. A holographic recording system, comprising: a light beam irradiationunit for irradiating an information beam and a reference beam so that aninterference pattern formed by the information beam and the referencebeam can be recorded as a diffraction grating into an informationrecording region in a holographic recording medium; a recording mediummoving unit for moving the holographic recording medium with respect tothe optical axes of the information beam and the reference beam; and anoptical chopper provided on the optical axes of the information beam andthe reference beam between the light beam irradiation unit and theholographic recording medium, the optical chopper moving the opticalaxes in a same traveling direction as, by substantially a same travelingdistance as, and in synchronization with the information recordingregion, when performing holographic recording into the informationrecording region by using the information beam and the reference beam.2. The holographic recording system according to claim 1, wherein: theoptical chopper comprises a chopper lens that travels in synchronizationwith and in a same direction as the information recording region; andthe chopper lens is formed so as to irradiate the information beam andthe reference beam from the light beam irradiation unit to be incidenton the information recording region, and to refract the information beamand the reference beam according to variations of incident positions onthe chopper lens to make exit optical axes pass through an identicalpoint, when performing the holographic recording.
 3. The holographicrecording system according to claim 2, wherein: the holographicrecording medium has a disc shape; the optical chopper comprises adisc-shaped chopper disc having substantially a same radius as theholographic recording medium and capable of rotating in synchronizationwith the holographic recording medium; the chopper disc has a pluralityof ring-shaped regions disposed at different positions in a radiusdirection; and the chopper lenses are discretely disposed in each of thering-shaped regions in the chopper disc, with identical length and pitchin a circumference direction, and are formed so as to make the referencebeam and the information beam pass through the chopper disc in athickness direction.
 4. The holographic recording system according toclaim 2, wherein the chopper lens is a cylindrical lens elongated in adirection orthogonal to a traveling direction of the holographicrecording medium.
 5. The holographic recording system according to claim3, wherein the holographic recording medium and the chopper disc arecoaxially and integrally provided so as to be freely rotated.
 6. Theholographic recording system according to any one of claims 1, furthercomprising: a CCD camera for forming a reconstructed holographic imageby a diffraction beam from the information recording region in theholographic recording medium, when a reconstructing beam is irradiatedto the information recording region through the chopper lens; and acompensating unit for compensating position offset of the reconstructedholographic image obtained by the CCD camera, caused by movement of thechopper lens.
 7. An optical chopper comprising: a chopper disc having arotatable disc shape and comprising a plurality of ring-shaped regionsdisposed at different positions in a radius direction; and a pluralityof chopper lenses that are discretely disposed in the ring-shapedregions in the chopper disc, with an identical length and pitch in acircumference direction, and refracts and transmits a beam incident fromone surface of the chopper disc to the other surface thereof, whereinthe chopper lens is formed such that, when the chopper lens is rotatedin a fixed direction at a constant speed together with the chopper disc,an exit optical axis of a laser beam incident from the one surface isrefracted so as to take a fixed position in a rotational direction in aspecific plane disposed on the other surface.
 8. The optical chopper ofclaim 7, wherein the chopper lenses are each a cylindrical lenselongated in the radius direction of the chopper disc.
 9. The opticalchopper of claim 7, wherein the chopper lenses and the chopper disc areintegrally formed of a transparent resin.
 10. The holographic recordingsystem according to claim 3, wherein the chopper lens is a cylindricallens elongated in a direction orthogonal to a traveling direction of theholographic recording medium.
 11. The holographic recording system ofaccording to claim 4, wherein the holographic recording medium and thechopper disc are coaxially and integrally provided so as to be freelyrotated.
 12. The holographic recording system of according to claim 10,wherein the holographic recording medium and the chopper disc arecoaxially and integrally provided so as to be freely rotated.
 13. Theholographic recording system according to any one of claim 2, furthercomprising: a CCD camera for forming a reconstructed holographic imageby a diffraction beam from the information recording region in theholographic recording medium, when a reconstructing beam is irradiatedto the information recording region through the chopper lens; and acompensating unit for compensating position offset of the reconstructedholographic image obtained by the CCD camera, caused by movement of thechopper lens.
 14. The holographic recording system according to any oneof claim 3, further comprising: a CCD camera for forming a reconstructedholographic image by a diffraction beam from the information recordingregion in the holographic recording medium, when a reconstructing beamis irradiated to the information recording region through the chopperlens; and a compensating unit for compensating position offset of thereconstructed holographic image obtained by the CCD camera, caused bymovement of the chopper lens.
 15. The holographic recording systemaccording to any one of claim 4, further comprising: a CCD camera forforming a reconstructed holographic image by a diffraction beam from theinformation recording region in the holographic recording medium, when areconstructing beam is irradiated to the information recording regionthrough the chopper lens; and a compensating unit for compensatingposition offset of the reconstructed holographic image obtained by theCCD camera, caused by movement of the chopper lens.
 16. The holographicrecording system according to any one of claim 5, further comprising: aCCD camera for forming a reconstructed holographic image by adiffraction beam from the information recording region in theholographic recording medium, when a reconstructing beam is irradiatedto the information recording region through the chopper lens; and acompensating unit for compensating position offset of the reconstructedholographic image obtained by the CCD camera, caused by movement of thechopper lens.
 17. The optical chopper of claim 8, wherein the chopperlenses and the chopper disc are integrally formed of a transparentresin.